1. Field of the Invention
This invention relates to an isolated preparation of human trophoblast stem cells and use thereof.
2. Description of the Related Art
In mammals, the earliest developmental decision specifies the trophoblast cell lineage. In mice, this lineage appears at the blastocyst stage as the trophectoderm, a sphere of epithelial cells surrounding the inner cell mass (ICM) and the blastoceol. After implantation, the ICM gives rise to the embryo proper and some extraembryonic membranes. However, the trophectoderm is exclusively restricted to form the fetal portion of the placenta and the trophoblast giant cells. The polar trophectoderm (the subset of trophectoderm in direct contact with the ICM) maintains a proliferative capacity and gives rise to the extraembryonic ectoderm (E×E), the ectoplacental cone (EPC), and secondary giant cells of the early conceptus. The rest of the trophectoderm ceases to proliferate and becomes primary giant cells. Studies in primary culture and chimeric mice have suggested that stem cells exist in the extraembryonic ectoderm which contribute descendants to the EPC and the polyploid giant cells. Further evidence indicated that maintenance of these stem cell-like characteristics was dependent on signals from the ICM and later from the epiblast, since diploid trophoblast cells transformed into giant cells when removed from the embryonic environment. However, the nature of the embryo-derived signal was not known and all attempts at routine long-term culture of mouse trophoblast stem cells have been unsuccessful (U.S. Pat. No. 6,330,349).
Stem cells have the capacity to divide and proliferate indefinitely in culture. Scientists use these two properties of stem cells to produce seemingly limitless supplies of most human cell types from stem cells, paving the way for the treatment of diseases by cell replacement. In fact, cell therapy has the potential to treat any disease that is associated with cell dysfunction or damage including stroke, diabetes, heart attack, spinal cord injury, cancer and AIDS. The potential of manipulation of stem cells to repair or replace diseased or damaged tissue has generated a great deal of excitement in the scientific, medical and/biotechnology investment communities.
U.S. Appl. No. 2003104616 disclosures that ES cells from various mammalian embryos have been successfully grown in the laboratory. Evans and Kaufman (1981) and Martin (1981) showed that it is possible to derive permanent lines of embryonic cells directly from mouse blastocysts. Thomson et al., (1995 and 1996) successfully derived permanent cell lines from rhesus and marmoset monkeys. Pluripotent cell lines have also been derived from pre-implantation embryos of several domestic and laboratory animal species such as bovines (Evans et al., 1990) Porcine (Evans et al., 1990, Notarianni et al., 1990), Sheep and goat (Meinecke-Tillmann and Meinecke, 1996, Notarianni et al., 1991), rabbit (Giles et al., 1993, Graves et al., 1993) Mink (Sukoyan et al., 1992) rat (Iannaccona et al., 1994) and Hamster (Doetschman et al., 1988). Recently, Thomson et al (1998) and Reubinoff et al (2000) have reported the derivation of human ES cell lines. These human ES cells resemble the rhesus monkey ES cell lines.
ES cells are found in the ICM of the human blastocyst, an early stage of the developing embryo lasting from the 4th to 7th day after fertilization. The blastocyst is the stage of embryonic development prior to implantation that contains two parts via.
1. Trophectoderm: outer layer which gives extra embryonic membranes.
2. Inner cell mass (ICM): which forms the embryo proper.
In normal embryonic development, ES cells disappear after the 7th day and begin to form the three embryonic tissue layers. ES cells extracted from the ICM during the blastocyst stage, however, can be cultured in the laboratory and under the right conditions proliferate indefinitely. ES cells growing in this undifferentiated state retain the potential to differentiate into cells of all three embryonic tissue layers. Ultimately, the cells of the inner cell mass give rise to all the embryonic tissues. It is at this stage of embryogenesis, near the end of first week of development, that ES cells can be derived from the ICM of the blastocyst.
The ability to isolate ES cells from blastocyst and grow them in culture seems to depend in large part on the integrity and condition of the blastocyst from which the cells are derived. In short, the blastocyst that is large and has distinct inner cell mass tend to yield ES cells most efficiently. Several methods have been used for isolation of inner cell mass (ICM) for the establishment of embryonic stem cell lines. Most common methods are natural hatching of the blastocyst, microsurgery and immunosurgery.
Expression and functional analyses indicated that FGF4 and fgfr-2 may be involved in trophoblast proliferation. The reciprocal expression domains of fgfr-2 and FGF4 suggested that the trophoblast could be a target tissue for an embryonic FGF signal. fgfr-2-null and FGF4-null mice show similar peri-implantation lethal phenotypes. This may result from defects in the ICM and its endoderm derivatives. However, it is also consistent with the possibility that FGF4 acts on the trophoblast through fgfr-2 to maintain a proliferating population of trophoblast cells. Support for this latter possibility is provided by recent studies showing that inhibiting FGF signaling blocked cell division in both the ICM and trophectoderm.
In humans, the inner cell mass (ICM) of blastocyst generates human embryonic stem (hES) cells at the earliest stage of embryogenesis (J. A. Thomson et al., Science, 282, 1145 (1998)). The hES cells appear approximately 4-5 days postfertilization with full self-renewal capacity and can yield all of the specialized cell phenotypes of the body. Human embryonic germ stem (hEG) cells, which are derived from fetal primordial germ ridge at 5-9 weeks post-fertilization, also possess pluripotency (M. J. Shambloff et al., Proc. Natl. Acad. Sci. USA, 95, 13726 (1998)). In vitro, both hES and hEG cells will spontaneously generate embryoid bodies (EBs) that consist of cell types from all three primary germ layers (M. Amit et al., Dev. Biol., 227, 271, (2000); M. J. Shambloott et al., Proc. Natl. Acad. Sci. USA., 98, 113, (2001)), giving an enormous potential to be used in cell-based therapies (K. Hochedlinger and R. Jaenisch, N. Engl. J. Med., 349, 275 (2003)).
Comparatively, research has paid less attention to the outer trophectoderm of blastocyst in humans. Most of the knowledge on hTS cells has been based on experiments on mice. In mice, trophectodermal subtypes initiate at peri-implantation to form three distinctive trophoblast cell layers. The trophectoderm overlying the ICM continue to divide and form the polar trophectoderm, which then grows into the extraembryonic ectoderm (E×E), where a diploid cell population is maintained with some develop into the mature chorioallantoic placenta (A. J. Copp, J. Embryol. Exp. Morphol., 51, 109 (1979)). This model presents E×E as a potential source of stem cells for the trophoblast lineage (J. Rossant and W. Tamura-L is, J. Embryol. Exp. Morphol., 62, 217 (1981)). In humans, research suggests that hTS cells may occur in the later stages of placental development rather than in the blastocyst (J. Rossant, Stem Cells 19, 477 (2001)). Another study suggests that no hTS cells exist, and that any possible existence of hTS cells would likely originate from the cytotrophoblast layer (T. Kunath et al., in: Trophoblast stem cells, chapter 12, in: Stem Cell Biology, D. R. Msrshak, R. L. Gardner, D. Gottlieb, Eds. (Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. 2001), pp. 267-287).
Other possible disadvantages of the existing cell lines are as follows:
1. Use of feeder cells for culturing the human embryonic stem cell (hESC) lines produces mixed cell population that require the embryonic stem cells (ESC) to be separated from feeder cell components and this impairs scale up.
2. ESC get contaminated by transcripts from feeder cells and cannot be used on a commercial scale. It can be used only for research purposes.
U.S. Appl. No. 2003104616 disclosures that Geron established a procedure where hESC line was cultured in the absence of feeder cells (XU et. al 2001). The hESC were cultured on an extracellular matrix in a conditioned medium and expanded in this growth environment in an undifferentiated state. The hESC contained no xenogenic components of cancerous origin from other cells in the culture. Also, the production of hESC cells and their derivatives were more suited for commercial production. In this process, there was no need to produce feeder cells on an ongoing basis to support the culture, and the passaging of cells could be done mechanically. However, the main disadvantage of this procedure is that the inner cell mass (ICM) is isolated by immunosurgery method, wherein the initial derivation of ESC is carried out using feeder layer containing xenogenic components. This raises the issue of possible contamination with animal origin viruses and bacteria.